Bitcoin fans keep saying "prove it is a bubble" because they do not believe it is - but this is a false question.
The real question all you bitcoin fans should be able to answer is: Prove that bitcoin is not a bubble.
A bubble is when the price of something is far higher than the fundamental value.
Bitcoin gives you ownership of nothing (a stock gives you ownership of tangible assets)
Bitcoin pays no dividend
Bitcoin gives you no right to future payments (bonds give you the right to payments)
Bitcoin gives you no patents or trademarks
Bitcoin gives you no legal rights
Given these facts, you Bitcoin fans need to prove that the fundamental value is different from zero (or close to zero) - otherwise it is a by definition a bubble
Actually, this is bitcoin's strength: its fundamental value IS zero, as you correctly said. As such, whether a bitcoin costs $1, or it costs $1M, is the same. There is no difference, the ratio is in any case infinite. This is why, given that there is strictly no reference value (a fundamental), bitcoin can have just any price, and any price is as good as any other. Whether you overpay something an infinite times, or an infinite times, is equivalent. That's entirely different if bitcoin were to have a small, but real, fundamental value. if bitcoin had something like, say, $10 of economic utility, paying $20 for a bitcoin seems not such a terribly bad idea, but paying $1000 for a bitcoin would be entirely objectively stupid, and the bubble would crash.
However, given that paying $1 for a coin is already *entirely* irrational, and we're used to that for many years, it is not more irrational to pay $1M for it than it is to pay $1 for it. This is why bitcoin keeps on bubbling, and the sky is the limit. Tullips only bubbled once. The South Seas only bubbled once. But bitcoin bubbled already 3 times: in 2011, in 2013 and in 2017. Because, exactly, its fundamental is zero.